1. Field of the Invention
The present invention relates to a liquid crystal display device having a sufficiently wide viewing angle which can be used in flat displays incorporated in, for example, portable information terminal devices, personal computers, word processors, amusement equipment and televisions viewed by a plurality of people, as well as display boards employing a shutter effect and display devices provided on windows, doors or walls.
2. Description of the Related Art
A conventional liquid crystal display device is known which includes a pair of substrates and a liquid crystal layer interposed therebetween. On one of the pair of substrates, scanning lines, data lines, switching elements and pixel electrodes are provided. The scanning lines and the data lines are provided so as to cross each other. Each of the switching elements is provided to be connected with the respective scanning line and data line. Each of the pixel electrodes is provided in a respective rectangular region enclosed by the scanning lines and the data lines to be in connection with the respective switching element.
In such a liquid crystal display device, each pixel electrode needs to be provided in the respective rectangular region without making a contact with the crossing scanning lines and data lines. Therefore, each pixel electrode is formed so that the periphery thereof is separated from the scanning lines and the data lines. As a result, the pixel electrode is made small, which results in a low aperture ratio.
A liquid crystal display device is proposed which is capable of providing a higher aperture ratio (Japanese Laid-Open Patent Publication No. 4-120516). The liquid crystal display device described in the above-mentioned publication includes a pair of substrates, a liquid crystal display layer interposed therebetween, scanning lines and data lines provided in a crossing manner on one of the pair of substrates, and switching elements provided in connection with the scanning lines and the data lines. Pixel electrodes are provided on an insulating layer covering the scanning lines, the data lines and the switching elements. Each of the pixel electrodes is connected with an output terminal of the respective switching element via a contact hole formed in the insulating layer.
In such a liquid crystal display device, the scanning lines and the data lines are provided on a different level from the pixel electrodes. Thus, even when the pixel electrodes are overlaid above the scanning lines and the data lines, the pixel electrodes are prevented from being shortcircuited therewith. Accordingly, the pixel electrodes can be made larger than those of a liquid crystal display device in which pixel electrodes, the scanning lines and the data lines are provided generally on the same level. Furthermore, an electric field occurring in a horizontal direction between each pixel electrode and the respective scanning and data lines can be minimized, thereby restraining defective orientation.
Although such a liquid crystal display device may be capable of obtaining a high aperture ratio, there still remains a problem of a poor viewing angle characteristic owing to the usage of TN (twisted nematic) mode. This is due to the following reasons.
Conventionally, TN or STN (super twisted nematic) liquid crystal display devices employing a nematic liquid crystal material are put into practical use as display devices utilizing an electro-optic effect. Such liquid crystal display devices require polarizing plates as well as an alignment layer. In such a liquid crystal device, liquid crystal molecules function as follows. As shown in FIG. 15A, the liquid crystal molecules are provided with a pretilt angle under an initial orientation state (i.e., a state with no voltage applied). As shown in FIG. 15B, the liquid crystal molecules rise in the same direction (rotate in the same direction) under application of a voltage. Under such a state, transmittance of the liquid crystal device varies relative to various viewing directions. For instance, the transmittance is different when viewed from directions represented by arrows A and B (hereinafter, simply referred to as "directions A and B"). Moreover, at a gray scale level, phenomena that remarkably deteriorate the display quality, such as inversion, may occur depending on the viewing angle. A cell of the liquid crystal display device under application of a saturation voltage is shown in FIG. 15C.
In order to minimize the inversion which occurs due to a narrow viewing angle characteristic of a liquid crystal display device, the following five types of liquid crystal display apparatuses with wider viewing angle characteristics have been proposed.
A first liquid crystal display apparatus includes two polarizing plates disposed so that the polarizing axes are perpendicular to each other with a polymer-dispersed liquid crystal device sandwiched therebetween (Japanese Laid-Open Patent Publication No. 4-338923 and Japanese Laid-Open Patent Publication No. 4-212928). Although the liquid crystal display apparatus has a great effect in improving the viewing angle characteristic thereof, the liquid crystal display apparatus has low merit for practical use since the liquid crystal display apparatus basically employs depolarization by scattering and thus the brightness is half as much as the brightness of a typical TN liquid crystal display apparatus.
A second liquid crystal display apparatus includes random liquid crystal domains for enhanced viewing angle characteristics. Such random liquid crystal domains are obtained by disturbing the orientation of the liquid crystal molecules, for example, by polymer walls or protrusions (Japanese Laid-Open Patent Publication No. 5-27242). Since the liquid crystal display apparatus includes random liquid crystal domains, the polymer material spreads over to picture element regions. Each picture element region is defined between each pixel electrode and a region of a counter electrode corresponding to the pixel electrode, including the pixel electrode and the region of the counter electrode corresponding to the pixel electrode. Moreover, disclination lines between the liquid crystal domains are randomly generated and such disclination lines are not eliminated even under application of a voltage. Consequently, the second liquid crystal display apparatus has a lower light transmittance under no applied voltage and a lower contrast under application of a voltage due to a lower black level.
A third liquid crystal display apparatus has a remarkably improved viewing angle characteristic since the liquid crystal molecules in each picture element region are oriented in an axially-symmetric manner within a polymer wall (Japanese Laid-Open Patent Publication No. 6-301015 and Japanese Laid-Open Patent Publication No. 7-120728), or alternatively since the liquid crystal molecules are oriented in a concentric or radial manner between the upper and lower substrates by subjecting the picture element regions to an alignment treatment (Japanese Laid-Open Patent Publication No. 6-324337 and Japanese Laid-Open Patent Publication No. 6-265902). As shown in FIG. 15D, the former liquid crystal display apparatus includes a pair of substrates 101 and 102 and liquid crystal molecules 109 in a liquid crystal region 108 enclosed by a polymer wall 107. The liquid crystal molecules 109 are oriented in an axially-symmetric manner by the polymer wall 107, and are given a pretilt angle under an initial orientation state. In FIG. 15D, reference numeral 110 denotes a disclination line. As shown in FIG. 15E, when a voltage is applied, the liquid crystal molecules 109 rise in the same direction (rotate in the same direction). Under such a state, a generally uniform transmittance is obtained when viewed from either directions A or B. A cell of the liquid crystal display device under application of a saturation voltage is shown in FIG. 15F.
The former liquid crystal display device, however, has a difficulty in controlling the pretilt angle and pretilt direction. On the other hand, the latter liquid crystal display device has a difficulty in performing concentric or radial orientation treatment for each of the picture element regions in terms of mass-production.
A fourth liquid crystal display apparatus includes an alignment film made of a crystalline polymer having spherulites. A spherulite is a spherical polycrystal in which a plurality of crystals are radially arranged starting from one point. Liquid crystal molecules are radially aligned along the spherulites, thereby enhancing the viewing angle characteristics (Japanese Laid-Open Patent Publication No. 6-308496).
A fifth liquid crystal display apparatus includes liquid crystal molecules randomly oriented by applying alignment films on substrates with an alignment treatment such as rubbing (Japanese Laid-Open Patent Publication No. 6-194655). When a voltage is applied to such a liquid crystal display apparatus, disclination lines are generated between liquid crystal domains due to a reverse tilt, thereby resulting in lower contrast.
The above-described liquid crystal display apparatuses with wide viewing angles have a region where a polarizing axis of the polarizing plates and an orientating axis do not align with respect to each other. As a result, the transmittance of such a liquid crystal display apparatus is reduced by at least about 10% compared with that of a conventional TN liquid crystal display apparatus. In the case of liquid crystal display device of an axially symmetrically aligned micro cell mode (hereinafter, simply referred to as an "ASM" mode) disclosed in Japanese Laid-Open Patent Publication No. 6-301015, lattice-like walls made of polymer are necessary to be provided outside the pixel electrodes in order to achieve an axially-symmetric orientation with high repeatability. Furthermore, convex portions are also necessary to be formed in a counter substrate (on which a color filter is provided) for accurate positioning of the axially-symmetric orientation axes. Accordingly, the number of steps for producing the liquid crystal display apparatus is increased. Moreover, if spacers, which are used to maintain uniform thickness of the cell, are dispersed in the picture element regions, axially-symmetric orientation is prohibited. In this regard, the spacers are necessary to be fixed outside the picture element regions.